First of all, at the moment he hasn't been proven guilty. Second, when and if he is, criminals aren't stripped of all of their rights.
The fact that the thief knows that the computer is stolen property and is not rightfully his makes a big difference. For example, if he bought the computer for $250 he might be suspicious that it was stolen (otherwise why such a good deal?), but it is theoretically possible in that situation to give him the benefit of the doubt. Of course, that's not the case here.